How to i extrude 4 faces at the same time
To extrude all the selected faces to a central point you can simply press E to extrude but then cancel the extrusion with Esc. This will leave the newly extruded geometry in the same place as the originating faces, but still selected. Then simply scale them to 0 with S0Enter.

The above actions will scale the etruded faces to a point - ie, it will taper together at the central point. To avoid this you can apply the scale in all but one direction - in your case the 'depth' of the object is in the Y direction so you could use S Shift+Y 0Enter.



Note that once you have scaled the faces together you might want to Remove Doubles to merge the vertices together.